Slice Tempeh in half (so it becomes half as thick), then cut into 4 quarters.
Heat oil in skillet until hot but not smoking and add Tempeh and brown on each side.
In a separate small bowl, whisk together the water, mustard and Tamari.
Pour mustard mixture over the browned Tempeh, cover, reduce heat to medium low and cook about 10 minutes, or until most of the liquid has evaporated.
Serve hot , garnished with green onions, or let cool and serve as a sandwich stuffing.